Reflective films comprising cholesteric liquid crystal materials have been proposed in prior art for a variety of uses, inter alia for use as broadband or notch polarizers, as colour filters in displays or projection systems, and for decorative purposes like e.g. for the preparation of coloured image films or cholesteric pigment flakes.
These films usually comprise one or more layers of a cholesteric liquid crystalline material with a helically twisted orientation, wherein the helix axis is perpendicular to the film plane, and show selective reflection of light.
The bandwidth .DELTA..lambda. of the waveband reflected by a reflective film as described above is depending on the birefringence of the mesogenic material .DELTA.n and the pitch of the molecular helix p according to the equation .DELTA..lambda.=.DELTA.n.times.p. Thus, the bandwidth among other factors is determined by the birefringence of the material.
For an application e.g. as broadband reflective polarizer in liquid crystal displays, it is desirable that the bandwidth of the reflective film should comprise a substantial portion of the visible wavelength range, whereas for an application as notch polarizer or as coloured reflective film e.g. for decorative or security applications, often films having a specific reflection colour are desired.
In particular broadband reflective polarizers, also known as circular polarizers, which are transmitting circularly polarized light of a broad wavelength band covering a large part of the visible spectrum, are suitable as polarizers for backlit liquid crystal displays.
If unpolarized light is incident on such a reflective polarizer, 50% of the light intensity are reflected as circularly polarized light with the same twist sense as that of the molecular helix, whereas the other 50% are transmitted. The reflected light is depolarized (or its sense of polarization is reversed) in the backlight of the display, and is redirected onto the polarizer. In this manner theoretically 100% of a given waveband of the unpolarized light incident on the reflective polarizer can be converted into circularly polarized light.
The circularly polarized light can be converted into linear polarized light by means of a quarter wave optical retarder and optionally also a compensation film.
A simple, but neither very effective nor economic way to provide a broadband reflective polarizer is to stack several reflective films with different reflection wavebands on top of each other. Recently reflective polarizers have been developed that comprise a liquid crystalline material with a helically twisted structure and a planar orientation, and are further characterized in that the pitch of the molecular helix is varying in a direction perpendicular to the layer, which leads to a large bandwidth of the reflected wavelength band.
Methods described so far for the preparation of broadband reflective polarizers from liquid crystalline precursors do have various drawbacks. The EP 0 606 940 (Broer et al.) discloses circular reflective polarizers with a bandwidth of up to 400 nm and their manufacture. This is realized by the exploitation of the diffusion of reactive mesogenes with different reactivity and chirality leading to a large variation of the cholesteric pitch, as disclosed in Broer et al. Nature, Vol. 378, pp. 467 (1995). However, this process is rather slow and in some cases even takes several minutes to complete. This is incompatible with most methods to fabricate polarizers on continuously moving substrates such as plastic films.
A process for the production of reflective films on plastic substrates is described in the WO 97/35219. Though this process is completed in the order of 15 to 30 seconds, and is thus faster than that used by Broer et al., it is nevertheless still relatively difficult with respect to the control of the resultant reflection wavelength and bandwidth of the reflective polarizer.
Furthermore, the methods described in the EP 0 606 940 and WO 97/35219 can only lead to spatially uniform characteristics of the reflective films, i.e. showing no variation of the pitch in lateral directions across the film. On the other hand, there are also applications where it is desired to have a reflective film with reflection characteristics that are spatially varying over the film, e.g. wherein different areas of the film show different reflection colours. These films are useful e.g. for information storage or as multicoloured images.
The GB 2,315,760-A discloses a polymerizable mesogenic composition that is thermochromic, i.e. it shows a change of the reflection colour upon temperature variation, and also discloses a method to prepare a multicoloured reflective film thereof, by coating the composition as a thin, oriented layer onto a substrate, selectively heating different regions of the layer to different temperatures (e.g. by means of a laser), so that they exhibit different reflection colours, and curing the different regions to fix the respective colour.
The method described in the GB 2,315,760-A, however, is still relatively complicated and time-consuming, as several heating and curing steps are required.
Consequently there w as a need for a method to prepare reflective films with better and more easy control both of the reflection wavelength and the bandwidth of the film, as well as for a method to produce reflective films with spatially varying reflection wavelengths, wherein these films could be used as reflective polarizers, colour filters, or as coloured films for information storage or in decorative or security applications.
Definition of Terms
In connection with reflective films and optical polarization, compensation and retardation films as d described in the present application , the following definition of terms are given.
The term `reflective film` as used in this application includes self-supporting, i.e. free-standing, films that show more or less pronounced mechanical stability and flexibility, as well as coatings or layers on a supporting substrate or between two substrates.
The term `helix axis perpendicular to the film plane` means that the helix axis is substantially perpendicular to the film plane, i.e. substantially parallel to the film normal. This definition also includes orientations where the helix axis is tilted at an angle of up to 2.degree. relative to the film normal.
The term `thermodynamically stable mesophase` means the state that is obtained upon polymerization of a polymerizable mesogenic material, where the system during polymerization has sufficient time to relax to give the thermodynamically stable, highly ordered equilibrium mesophase of the polymerized material. The thermodynamically stable, equilibrium mesophase of the polymerized material can be achieved erg. by polymerizing the mesogenic material in solution, or at low polymerization rates or to low molecular weights.
The term `homeotropic orientation` means that the optical axis of the film is substantially perpendicular to the film plane, i.e. substantially parallel to the film normal. This definition also includes films wherein the optical axis is slightly tilted at an angle of up to 2.degree. relative to the film normal, and which exhibit the same optical properties as a film wherein the optical axis is exactly parallel to the film normal.
The terms `tilted structure` or `tilted orientation` means that the optical axis of the film is tilted at an angle between 0 and 90 degrees relative to the film plane.
The term `splayed structure` or `splayed orientation` means a tilted orientation as defined above, wherein the tilt angle additionally varies monotonuously in the range from 0 to 90.degree., preferably from a minimum to a maximum value, in a direction perpendicular to the film plane.
The term `planar orientation` means that the optical axis of the film is substantially parallel to the film plane. This definition also includes films wherein the optical axis is slightly tilted relative to the film plane, with an average tilt angle throughout the film of up to 1.degree., and which exhibit the same optical properties as a film wherein the optical axis is exactly parallel to the film plane.
In case the reflective polarizers and homeotropic, tilted, splayed, planar and twisted retardation and compensation films as defined above comprise uniaxially positive birefringent liquid crystal material with uniform orientation, the respective orientation of the optical axis corresponds to the orientation direction of the main molecular axes of the mesogens of the liquid crystal material.
The minimum and maximum wavelengths of the waveband reflected by an inventive reflective film, i.e. the edges of the band, in this application are not given as the values for half the values of the maximum of the bands. For practical reasons the minimum and maximum wavelengths are defined as those wavelengths on the given flank where the curve has the steepest slope in absolute values, compare FIGS. 5 to 8. The bandwidth is simply given as the difference between minimum and maximum wavelength. The central reflection wavelength also called short reflection wavelength or wavelength of reflection is given as the arithmetical average of the minimum and maximum wavelength.